1. Field of the Invention
The present invention relates generally toner formulations including silica blends utilizing large colloidal silica sized 90 nm to 120 nm and having a conductivity of less than 20 μS/cm in combination with medium size silica particles sized 30 nm to 60 nm.
2. Description of the Related Art
Toners for use in electrophotographic printers may include two primary types, namely chemically prepared toners and toners made by a mechanical grinding process. Chemically prepared toners may have significant advantages over toners made by a mechanical grinding process. In a mechanical grinding process, particle breakage may be difficult to control and minimize. Also, the shape of mechanically ground particles may be more irregular than chemically prepared toner particles. Hence, the particle size distribution of mechanically ground toner particles may be relatively broader than for chemically prepared toner particles.
There are several types of chemically prepared toner, depending on the process used to make the chemically prepared toner. Chemically prepared toner may generally be classified as a suspension toner, an emulsion aggregation toner, a dispersion toner, or a chemically milled toner. Of the foregoing, a suspension toner is made by the simplest process. However, the shape of a suspension toner may be limited to spherical, and the size distribution of such toner may be dependent on how the toner ingredients are dispersed in a monomer used to make the toner. On the other hand, an emulsion aggregation toner may involve a more complex process. However, the emulsion aggregation process may provide a toner having a relatively narrower size distribution, and the shape and structure of the toner particles may be more controllable.
In a typical emulsion aggregation chemically prepared toner process, the toner components may include pigment, wax, and a latex binder which may be dispersed by use of surfactants. The toner may optionally include a charge enhancing additive or charge control agent.
One of the more important requirements of printers is print quality. In color laser printers, resolution may be very critical. Higher or better resolution may be achieved by using toner having a small particle size. Small particle size may be more difficult to achieve from a conventional toner processing technique, due to limitations in mechanical extruding/grinding. By preparing the toner chemically, a smaller particle size may be more readily obtained. As noted above, there may be at least two processes to prepare a chemical toner, i.e. a suspension polymerization, or an emulsion agglomeration process.
Toner may consist of a base particle and surface-borne extra particulate additives. These extra particulates may serve a variety of functions, may generally be submicron in size, and have a very high surface area. The high surface area of the extra particulate additives and morphology of the toner may tend to promote adhesion between the extra particulate additives and the toner particles. Thus, toner particles may be treated with smaller size particulate additives such as silicas, titanias, aluminas, other metal oxides, metal carbides or organic microspheres. The addition of these particulate additives may improve the charge stability, flow characteristics, and environmental stability of toner. Treatment of toner particles with additives may render the toner more stable at various temperature and humidity conditions. As the particulate additives may be physically held on the surface of the toner particle, there may be some additives which may be more difficult to dislodge from the toner particle, thereby affecting such toner properties as filming, charging, mass flow, and, in general, print quality.